1. Technical Field of the Invention
This invention relates generally to wireless communication devices and more particularly to local oscillators with a wireless communication device.
2. Description of Related Art
It is well known that a wireless transmission originates at a transmitter of one wireless communication device and ends at the receiver of another wireless communication device. The structure of the wireless transmission is dependent upon the wireless communication standard, or standards, being supported by the wireless communication devices. For example, IEEE 802.11a defines an orthogonal frequency division multiplexing (OFDM) wireless transmission protocol that included eight 20 MHz spaced channels in the lower band (e.g., 5.15 gigahertz to 5.35 gigahertz) and four 20 MHz spaced channels in the upper band (e.g., 5.725 gigahertz to 5.825 gigahertz). Each channel may include 52 sub-carriers, 48 of which carry data based on a sub-carrier modulation mapping. Such sub-carrier modulation mapping includes binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM) or 64-QAM.
Typically, during a wireless transmission, only one channel carries valid data. Accordingly, the receiver tunes its one or more intermediate frequency (IF) stages such that the desired channel is centered within the filter response of the receiver to convert to baseband. As such the desired channel is recaptured as a baseband signal and subsequently decoded in accordance with the sub-carrier modulation mapping to obtain the transmitted data.
If, from one wireless transmission to the next, the channel is changed, the receiver needs to adjust its IF stage, or stages, in particular, change the frequency of the local oscillation, to receive the new channel. With most local oscillation designs, it takes hundreds of micro seconds to thousands of micro seconds to adjust from one local oscillation frequency to another. An improvement on this is disclosed in “Low Phase Noise, Fast Settling PLL Frequency Synthesizer” part number ADF4193 by Analog Devices.
For 802.11a applications, the specification requires channel switching to take less than 1 micro-second. As such, adjusting the local oscillation using conventional technique for channel switching in an 802.11a receiver and/or an 802.11 transmitter is unacceptable. For multiple transmission path communications, the situation is exacerbated and can require channel changes on the order of several micro-seconds.
Therefore, a need exists for a fast multiple transmission path switching local oscillation module for wireless communication devices.